Seasonal trends of legacy and alternative flame retardants in river water in a boreal catchment.

Boreal forests store large amounts of atmospherically deposited (semi-)persistent organic pollutants (POPs). The terrestrial POPs may be exported to streams and rivers through processes that are heavily impacted by seasonality. In this screening study, concentrations of 4 legacy and 45 alternative flame retardants (FRs) were determined in the dissolved and particulate phase in streams within a relatively pristine boreal catchment in northern Europe (Krycklan Catchment Study; 3 sites) and in rivers more impacted by human activities further downstream towards the Baltic Sea (3 sites). The sampling included the main hydrological seasons (snow-free, snow-covered, and spring flood) and was conducted during two consecutive years (2014-2016). Of the 49 analyzed FRs, 11 alternative halogenated FRs (HFRs), 13 alternative organophosphorus FRs (OPFRs), and 4 legacy polybrominated diphenyl ethers (PBDEs) were detected in at least one sample. The average bulk (dissolved + particulate) concentrations of ∑FRs (including all sites) were highest for ∑HFRs (38 ± 70 ng L-1), followed by the ∑OPFRs (3.9 ± 4.9 ng L-1) and the ∑PBDEs (0.0040 ± 0.016 ng L-1). Bulk concentrations of HFRs, OPFRs, and PBDEs were highly variable with season and sampling location, e.g., during spring flood, bulk concentrations were up to 600 times, 3.7 times, and 4.9 times higher for HFRs, OPFRs and PBDEs, respectively, than during periods of lower flow. Bulk concentrations of ∑OPFRs, were elevated at all sites ~6 days before the actual start of the spring flood in 2015, suggesting that hydrophobicity fractionation had occurred within the snowpack. Similar to previous studies of other POPs in the same headwater catchment, there was a general trend that levels of ∑FRs were higher at the mire site than at the forested site. Annual fluxes of FRs were found to be ~15 times higher downstream the city of Umeå compared to at the outlet of the pristine catchment. This study should be regarded as a screening study considering the large number of diverse FRs analyzed and variability in the results.